This invention pertains to apparatus and methods for polishing a workpiece such as a semiconductor wafer on which one or more thin patterned layers have been applied. More specifically, the invention pertains to such apparatus and methods that monitor, during polishing or other process step that imposes a time-varying change in thickness of such a patterned layer, the extent of thickness change imparted to the layer to determine whether a desired endpoint has been reached. The invention also pertains to apparatus and methods that measure layer and film thicknesses, especially the thickness of a layer or film on the surface of a workpiece having multiple thin-film layers, and minute surface irregularities of such films.
Semiconductor devices continue to be developed with no apparent upper limit on the feature density of such devices. Many obstacles associated with higher feature densities have been overcome using various technologies and methods.
One significant lingering problem has been achieving, during fabrication of the semiconductor device, satisfactory planarization of wafer surfaces having comparatively large area. As the integration density of devices has increased, the wavelength used for microlithography has tended to decrease with a concomitant need to form various features in different layers in the thickness dimension of the device. Reducing the wavelength of light used for microlithography tends to lead to a reduced depth of focus of the projection-optical system used for microlithography. Hence, there is an increasing demand for more precise planarization of wafer workpieces, at least over the exposure area of the wafer, between sequential microlithography steps. Also, as greater demands are made of so-called xe2x80x9cinlaysxe2x80x9d (i.e., implantation of metal electrode layers, plugs, or damascene in the thickness dimension), the need to remove excess metal and achieve planarization after forming each layer is increased.
With improvements in techniques used to form layers on a semiconductor wafer or the like, various methods have been proposed and implemented for achieving at least localized planarization of each layer before applying the next layer. Demand for continued improvements in planarization methods is escalating.
A commonly used planarization technique employing surficial polishing to planarize relatively large areas on a workpiece (generally termed herein a xe2x80x9cwaferxe2x80x9d) is termed xe2x80x9cCMPxe2x80x9d (Chemical Mechanical Polishing or Chemical Mechanical Planarization). CMP removes surface irregularities on the wafer by combining physical polishing with chemical action, and is effective for polishing insulating layers or conductive layers. In CMP, a polishing agent in the form of a slurry is used in which granules of an acid or alkaline abrasive (e.g., silica, alumina, and cerium oxide commonly are used) are suspended in a liquid in which the abrasive granules are at least partially soluble. Polishing proceeds by applying an amount of the slurry and an appropriate polishing surface to the wafer surface and using relative motion of the polishing surface and wafer surface. The surface of the wafer is polished uniformly by keeping the pressure and the speed of the relative motion uniform across the wafer surface.
CMP exhibits certain problems, however. One important problem is achieving accurate detection of when a polishing step should be ended (i.e., detection of the optimal polishing endpoint). There is an urgent need for accurate detection of the polishing endpoint in-situ, i.e., detecting the extent of polishing while polishing is ongoing.
One conventional method for detecting the polishing endpoint utilizes changes in the torque of a motor rotating the wafer or polishing surface during polishing; e.g., detecting a change in friction when polishing has progressed depthwise sufficiently to encounter a layer beneath the target polishing layer. Unfortunately, this method is effective only in detecting when the polishing has progressed into a different layer underlying the layer being polished. Also, this method suffers from insufficient accuracy and precision.
Another conventional method involves measurement by optical interference of the thickness of a layer during polishing of the layer. According to one approach, an optical path is provided in a polishing pad through which a light beam is irradiated on the wafer surface being polished. According to another approach, a wafer-penetrating (e.g., infrared) light beam is directed through an optical path provided in the wafer carrier that contacts the reverse surface of the wafer while the obverse surface of the wafer is being polished. Alternatively, the light beam can be directed through an optical path provided in the polishing pad.
In the interference techniques, changes in an interference pattern in reflected light are monitored over time. I.e., the interference pattern changes as the thickness of the subject layer changes. The layer thickness or amount of polishing of the subject layer that has occurred can be calculated from such data.
Employing interference to measure layer thickness (i.e., measurement of time-varying changes in an interference pattern produced using reflected laser light) suffers from unreliability and errors caused by variations in measurement position.
For example, the surface being polished normally does not always have optically uniform characteristics. For example, FIG. 1(a) shows a wafer 1 (i.e., semiconductor wafer with multiple applied layers) at time of beginning polishing. The wafer 100 comprises metal conductive traces 102 or the like embedded in an insulating layer 101. The goal of polishing is to remove the protruding portions 103 of the insulating layer 101, thus planarizing the surface of the wafer 100. In FIG. 1(b), a portion of a metal layer 102 superposed on an insulating layer 101 is removed. With wafers such as shown in FIGS. 1(a) and 1(b), light passing through or reflected from the wafer surface would be influenced in many ways. For example, regions on which the metal layer 102 is present can interrupt transmitted light and produce more reflected light than regions lacking the metal layer 102.
Further with respect to FIG. 1(a), changes in layer thickness are normally limited to the protruding portions 103. Thus, variations in the amount of transmitted or reflected light can be measured only from the protruding portions. Furthermore, when surface irregularities exist, producing a satisfactory light signal indicative of layer thickness is impossible using conventional methods.
Thus, certain conventional methods (including conventional methods that utilize interference) suffer from the problem of excess noise entering the light signal, especially when the wafer contains metal conductive traces or the like.
In situ measurements are performed while polishing is ongoing and thus require that the wafer be moving while the measurements are obtained. Such a situation generates complex signals that tend to exhibit more instability (e.g., intensity of reflected light, etc.) than when the wafer is stationary. This is due in part to the fact that the light must pass through the polishing slurry which tends to disperse the light to varying degrees when in motion.
Another problem with contemporary layer-thickness measuring methods using interference is that the surface of the wafer to be polished inevitably includes minute irregularities. FIG. 2 shows a typical example of such irregularities in a wafer ready for polishing. The wafer comprises a Si substrate 105 and a SiO2 layer 106. Within the SiO2 layer 106 are metal conductors 107. Light reflected from the surface of such a wafer comprises the following components: interference light (designated xe2x80x9caxe2x80x9d) generated by reflection from concave portions 108 and convex portions 109, and interference light (designated xe2x80x9cbxe2x80x9d) generated from further interference of the xe2x80x9caxe2x80x9d interference light, including a diffracted light component arising from phase changes in the concave and convex portions 108, 109.
Conventional layer-thickness detection using optical interference assumes a flat irradiated surface. Layer-thickness data is obtained from the profile of the peaks and valleys of the interference pattern generated by reflected light and certain optical constants (the refractive index and an absorption constant) of the layer. Such calculations are performed with the assumption that components xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d as summarized above are not present. In such a scheme, the actual presence of the xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d components contributes an error factor to the calculations.
Hence, it is conventionally necessary to make measurements in a planar section of the wafer that lacks surficial irregularities. Unfortunately, it is difficult and impractical to find a planar section for performing measurements during polishing.
Another conventional technique for measuring layer thickness is ellipsometry as disclosed in Japanese Laid-Open Patent Document No. Hei 7-193033. This technique is diagrammed in FIG. 3. A light flux emitted from an optical-fiber light source 115 passes through a collimator lens 120. The collimated light passes through a polarizer 121 that transmits the linearly polarized light component. The linearly polarized light then passes through a quarter-wave plate 122 and is converted into elliptically polarized light. The elliptically polarized light passes through a half-wave plate 123 that changes the orientation of the elliptical axis. The elliptically polarized light is incident on the patterned surface 116 of a wafer 117, is reflected from the surface 116, and passes through a filter 124 that transmits linearly polarized light. The linearly polarized light passes through a condenser lens 125 to a detector 126. The polarized state of the light flux reflected from the patterned surface 116 changes depending upon the thickness of the patterned surface 116 and the polarization state of the incident elliptically polarized light. The output of the detector 126 comprises data, obtained while rotating the half-wave plate 123, that can be processed to determine the thickness of the patterned surface 116.
Ellipsometrically measuring layer thicknesses of wafers comprising multiple die patterns on separate regions of the wafer surface is inaccurate because of an adverse effect of the multiple dies on the wafer. One conventional approach for minimizing such effects involves actual recognition of the pattern(s) on the wafer. Based on such recognition information, a beam of inspection light is directed to a particular desired location on the wafer surface. However, this scheme requires that an optical system be capable of selectively directing the beam in a particular direction, which has been elusive to achieve. Also, such a configuration has proven to be incapable of inspecting layer thicknesses on a wafer while the wafer is rotating rapidly.
Because of the shortcomings of conventional technology for monitoring polishing in situ, one conventional alternative approach involves controlling polishing based on polishing time. Such a basis is inaccurate and imprecise, and the wafers typically must be fabricated with layers that are thicker than necessary.
Another conventional approach is to perform layer-thickness measurements off-line after polishing. A typical example of such an approach involves transporting the wafer to a measuring device and performing optical film-thickness measurements there. The results of the measurements are used in feedback control of the polishing step. If the measurement indicates that more polishing is needed, the wafer is sent back for further polishing. The measurement method typically includes either washing and drying the wafer before performing the measurements or performing the measurements underwater. (If the polished wafer is dried before it has been washed, the polishing material can become impossible to remove.)
Most polishing pads are sufficiently stable to allow polishing to be briefly stopped for the wafer to be transported for off-line measurements. However, off-line measurements typically require that many time-consuming, cumbersome, and error-engendering steps be performed including removing the wafer from the polishing head, transporting the wafer to a measuring device, aligning of the wafer in the measuring device, performing the measurement, and then transporting the wafer back to the polishing head. Also, considerable time is required from the moment polishing is interrupted to performing the measurements, which can destroy meaningful feedback.
An object of the present invention is solve the problems, discussed above, of the prior art by providing methods and apparatus for measuring a thickness of a layer, wherein the layer is subjectable to a process that causes a time-varying change in thickness and/or planarity. For example, the methods and apparatus of the invention are especially applicable to wafer polishing as encountered in semiconductor device manufacturing, and fulfill a need to detect accurately and easily, either during or after polishing, the extent of polishing imparted to a layer (e.g., an insulating layer or a conductive layer) on a wafer substrate. The methods and apparatus are especially applicable to measuring thickness of such layers that are patterned or otherwise have minute surficial irregularities.
According to a first aspect of the invention, apparatus are provided for measuring the thickness of a layer on a workpiece comprising the layer applied to a planar substrate (e.g., semiconductor wafer). According to a preferred embodiment, the apparatus comprises a probe light source that produces a beam of probe light that preferably has multiple wavelengths. The probe light source is operable to direct the beam of probe light to be incident on the layer and produce a signal light from reflection of the probe light from or transmission of the probe light through the layer. The apparatus also comprises a light detector situated so as to receive and detect multiple wavelengths of the signal light and produce a corresponding electronic signal encoding data regarding intensity at various wavelengths of the detected signal light. A processor preferably is connected to the light detector. The processor is configured (e.g., programmmed) to calculate, from the electronic signal, a spectrum of intensity versus wavelength. The spectrum provides data indicating the thickness and/or planarity of the layer.
An example embodiment of the apparatus summarized above comprises first, second, third, fourth, and fifth lenses. The first, second, and third lenses are situated between the probe light source and the workpiece so as to refract the probe light propagating from the probe light source to the workpiece. The fourth and fifth lenses are situated between the workpiece and the light detector so as to refract the signal light propagating from the workpiece to the layer. A beamsplitter is located between the second and third lenses. Probe light propagating from the second lens passes through the beamsplitter to the third lens, and signal light propagating from the workpiece through the third lens is reflected by the beamsplitter to the fourth and fifth lenses. A diffraction grating is situated between the fifth lens and the light detector. Preferably, a beam-clipping aperture (i.e., an aperture that trims the transverse profile of the probe light beam) is situated between the second and third lenses, wherein the second lens is a collimating lens. Also, a pinhole aperture is situated preferably between the fourth and fifth lenses, wherein the fourth lens is a condenser lens.
According to another aspect of the invention, apparatus are provided for measuring the extent of surficial polish of a patterned layer on a workpiece comprising a planar wafer on which the patterned layer has been applied. A preferred embodiment of such an apparatus comprises a polishing assembly that holds the workpiece and subjects the patterned layer to polishing that removes at least portions of the patterned layer as the patterned layer is being planarized by the polishing. The apparatus also comprises a polishing-extent measuring assembly that comprises a probe light source, a light detector, and a processor. The probe light source produces a beam of probe light having multiple wavelengths and that directs the beam of probe light to be incident on a patterned region of the patterned layer. The light detector is situated so as to receive and detect signal light produced by either transmission of probe light through or reflection of probe light from the patterned region of the patterned layer. The light detector is operable to detect multiple wavelengths of the signal light and produce an electronic signal including data regarding light intensity at various wavelengths of detected signal light. The processor, to which the light detector is connected, is configured to calculate, from the electronic signal, a spectrum of intensity or transmittance (depending upon whether the probe light was reflected from or transmitted through, respectively, the patterned layer) versus wavelength. Such a spectrum provides data indicating the extent of polishing of the patterned layer.
The polishing assembly preferably comprises a wafer carrier that holds the wafer during polishing, and a polishing plate. A polishing pad is attachable to the polishing plate.
The processor is preferably configured to calculate a spectrum, whenever the probe light reflects from or passes through the patterned region of the patterned layer, that exhibits at least one maximum or minimum. The processor also preferably monitors at least one of the following in determining the extent of polishing of the patterned layer: (1) an appearance or disappearance of a maximum or minimum in the spectrum; (2) a change in a wavelength at which a maximum or minimum in the spectrum; and (3) a change in an intensity at a particular wavelength at which a maximum or minimum is located in the spectrum. Such events provide a stable signal that is reproducible from one similar workpiece to the next; the magnitude of such events is proportional to corresponding changes in the amounts of reflected or transmitted light and is not easily affected by such factors as interspersed polishing slurry, a lack of uniformity in layer thickness, or irregularities in the surface of the patterned layer or in its interface with the substrate or a deeper layer. If desired, the results obtained with a current workpiece can be compared by the processor to data obtained from a previous workpiece. Also, the extent of polishing of the patterned layer can be measured indirectly from data concerning the initial thickness of the workpiece and the thickness at a particular time during polishing.
Generally speaking, in determining a xe2x80x9cpolishing endpointxe2x80x9d (i.e., a desired thickness of the subject layer at which polishing should stop), the spectral reflectance or spectral transmittance curves typically include features that differ significantly from the spectral reflectance or spectral transmittance curves, respectively, produced before reaching the polishing endpoint. Thus, by measuring and/or calculating data pertaining to such features, the polishing endpoint can be detected accurately.
The probe light source preferably produces a beam of probe light having a transverse area larger than an area of a die on the patterned surface. The patterned surface of a wafer to be polished typically comprises multiple dies each comprising a large number of minute elements or xe2x80x9cfeatures.xe2x80x9d Thus, on the feature level, the surface of the patterned layer is not uniform. Thus, whenever the transverse profile of the probe light is substantially smaller than a die, the signal light produced by reflection or transmission of the probe light is influenced by the minute features, and exhibits changes depending upon the locus of incidence of the probe light. This can produce undesirable noise. By making the xe2x80x9cdiameterxe2x80x9d of the probe light beam substantially larger than a feature, the signal light is much more uniform regardless of the locus of incidence. This allows the light detector to produce a more stable signal.
In view of the above, the light detector and processor preferably produce the spectrum and thus determine the extent of polishing whenever the probe light is incident on a patterned region of the patterned layer. The principle behind the detection of the extent of polishing or the polishing endpoint makes it possible to detect the interaction of probe light with multiple layers in the patterned regions. For this reason, when the probe light is incident on a non-patterned region, obtaining a signal having sufficient data to determine layer thickness is more difficult. Hence, more accurate measurements are obtained when the probe light is incident on patterned regions.
According to another embodiment, the polishing plate and pad define a window that is transmissive to the probe light so as to allow the probe light to pass through the window to the patterned surface as the patterned surface is being polished by the polishing pad. Since polishing normally is performed by rotating the wafer (held by the wafer carrier) against the polishing pad (attached to a rotating polishing plate), the window allows measurements to be obtained in situ (i.e., while polishing is ongoing).
Another scheme for obtaining in situ measurements is to extend a portion of the wafer beyond an edge of the polishing pad. In such an instance, the probe light can be incident on the portion of the wafer extending beyond the edge.
The probe light preferably comprises multiple wavelengths. Upstream of the light detector can be a filter that selects one or more specific wavelengths of signal light for routing to the light detector. Such a scheme can permit faster detection of polishing extent compared to routing all probe-light wavelengths to the light detector.
If the patterned surface defines convex features and concave features, excellent measurement results can be obtained if the probe light is incident on the patterned surface at an angle of incidence xcex8 greater than zero. If H is an elevational difference between the convex and concave features and d is the width of the concave features, then preferably xcex8 greater than tan(xe2x88x921d/2H) to eliminate from the signal light a substantial amount of light reflecting from the concave features. This allows the layer thickness to be determined based on the same principle as when the subject layer surface is planar.
An apparatus as described above having a polishing assembly and a polishing-extent measuring assembly also can comprise a robot arm for conveying the workpiece from an entry-side standby position (e.g., an entry-side standby stage) to the polishing assembly for polishing, and from the polishing assembly to an exit-side standby position (e.g., an exit-side standby stage) to await downstream processing. In such a configuration the polishing-extent measuring assembly can be incorporated in the robot arm to permit a determination of the extent of polishing of the patterned layer to be made either as the robot arm is transporting the workpiece or when the robot arm has placed the workpiece on the polishing assembly.
If the apparatus comprises an exit-side standby stage, the polishing-extent measuring assembly can be incorporated in the exit-side standby stage. Such schemes avoid possible measurement instability associated with in situ measurements, and permit measurements to be made sooner (with correspondingly faster feedback to the polishing assembly) than with off-line measurements of layer thickness.
As an alternative to incorporating the polishing-extent measuring assembly in a robot arm, the assembly can be incorporated in the wafer carrier used to hold the workpiece during polishing and to urge the patterned surface of the workpiece against the polishing pad. In such an instance, the polishing-extent measuring assembly measures the extent of polishing of the patterned surface while the workpiece is being held by the wafer carrier. Preferably, the processor is operable to modify a condition of polishing of the patterned layer in response to a particular extent of polishing of the patterned layer as determined by the polishing-extent measuring assembly.
According to another aspect of the invention, an apparatus is provided for measuring the thickness of a layer of a workpiece consisting of a planar substrate and multiple layers applied to a surface of the substrate, wherein at least one of the layers has minute surface irregularities. Such an apparatus comprises a source of a beam of probe light, a probe-light optical system, a light detector, and a processor. The probe-light optical system directs a beam of probe light (produced by the probe light source) to be incident, at an incidence angle, on a surface of an outermost layer of the multiple layers and reflect from the surface as signal light. The signal light reflects from the surface at an angle equal to the incidence angle, wherein the surface irregularities cause the signal light to exhibit an interference. The light detector receives the signal light, measures a pattern of the interference of the signal light, and produces from the received signal light a corresponding electrical signal. The processor, which is connected to the light detector, is configured to measure the interference pattern and produce a spectrum having a characteristic indicative of the thickness of the layer.
According to another aspect of the invention, an apparatus is provided for measuring the thickness of a patterned surface layer on a semiconductor wafer, wherein the patterned surface layer comprises multiple dies each defining a circuit pattern for a semiconductor device. The apparatus comprises a probe-light optical system and a light detector. The probe-light optical system directs a probe light flux at the patterned surface layer, and adjusts the transverse profile of the probe light flux such that the probe light flux, when incident on the patterned surface layer, illuminates an area on the patterned surface layer equal in area to an integral multiple of one die and having sides parallel to the die. The probe light can be either transmitted by or reflected from the illuminated area so as to produce a corresponding signal light exhibiting an interference. The light detector receives the signal light, measures a characteristic of the interference of the signal light, and produces from the received signal light a corresponding electrical signal having a characteristic indicative of the thickness of the patterned surface layer. The apparatus can comprise a processor connected to the light detector. The processor measures the interference pattern and produces a spectrum having a characteristic indicative of the thickness of the layer.
The probe-light optical system of the embodiment summarized above can be used to adjust the transverse profile of the probe light incident on the patterned surface layer to have a profile exhibited either by one die or by multiple adjacent dies. The probe-light optical system can comprise a visual field aperture that clips the probe light flux to the desired transverse profile.
The visual-field aperture can be adjustable.
To such end, the visual-field aperture can be defined by a rotary plate defining multiple differently sized apertures through which individual apertures the probe light flux can selectively pass. Alternatively, the visual-field aperture can be defined by at least one laterally movable plate.
An optical element (e.g., a diffraction grating) can be situated between the light detector and the patterned surface layer. The optical element is used to separate from each other various diffraction orders of the signal light for selective delivery to the light detector. An aperture can be placed selectively downstream of the optical element to select one or more particular diffraction orders of signal light for propagation to the light detector. Such an aperture can be a pinhole aperture.
A condenser lens can be situated downstream of the patterned surface layer to receive the signal light propagating from the illuminated area of the patterned surface layer and condense the signal light at a focal point. A mirror defining an aperture can be situated at the focal point. Such a mirror is situated such that a first selected diffraction order of the signal light passes through the aperture to a first light detector, and a second selected diffraction order of the signal light is reflected by the mirror to a second light detector.
The apparatus can further comprise a condenser lens situated downstream of the patterned surface layer. The condenser lens is used to receive a selected order of diffracted signal light and condense the received signal light at a focal point. An optical fiber having an entrance terminus can be located at the focal point so as to retrieve the selected diffraction order of signal light and deliver the retrieved light to the light detector.
The probe-light optical system in the apparatus summarized above can include a probe light source. The probe light source can be used to produce a probe light having a certain wavelength bandwidth. In such an instance, the light detector can produce, from the characteristic of interference of the signal light, an electrical signal encoding a distribution of spectral intensity of the signal light flux over the bandwidth.
Alternatively, a probe light flux can be used that produces a substantially monochromatic probe light that is reflected from the patterned surface. In such an instance, the light detector can produce an electrical signal encoding an intensity distribution of the signal light flux.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.